The Intergovernmental Panel on Climate Change (also known as the IPCC) has linked many of these changes in climate to an increase in greenhouse gas emissions from human activities, which was documented in a 2007 assessment report compiled by thousands of scientists over decades of research and debate.

Regardless of their causes - whether you believe in anthropogenic drivers, like fossil fuels from power plants and cars or not — the observed changes in climate are scientific facts that have grave implications for the future of natural and human systems.

Global temperature trends estimated by four different research groups all show a warming of the Earth over the past century, with particularly rapid increases through the past few decades.

The groups are the National Oceanic and Atmospheric Administration, NASA, Britain’s Hadley Center and Climatic Research Unit (Univ. of East Anglia) and Japan’s meteorological agency.
Michael Schlesinger / University of Illinois via <a href="http://dotearth.blogs.nytimes.com/2009/11/30/more-on-the-climate-files-and-climate-trends/">DotEarth</a>

Since 1960, glaciers worldwide (not including the ice sheets in Antarctica and Greenland) have lost more than 2,000 cubic miles of water, contributing to observed changes in sea level rise.

Since 1900, seasonally frozen ground has decreased by 7 percent (up to 15 percent in the spring) in the Northern Hemisphere. The time that lakes stay frozen has generally decreased at an average rate of one to two days per decade since the mid-1800s.

The portion of North America covered by snow has generally decreased since 1972. The average extent for the 1970s (1972 to 1979) was 3.43 million square miles, compared with 3.3 million for the 1980s, 3.21 million for the 1990s, and 3.18 million from 2000 to 2008.

Since 1901, global precipitation has increased at an average rate of 1.9 percent per century (specifically in eastern parts of North and South America, northern Europe, and northern and central Asia), while precipitation in the lower 48 states has increased at a rate of 6.4 percent per century. Climate change will also cause some areas to experience decreased precipitation such as in the Sahel, the Mediterranean, southern Africa, and parts of southern Asia.

Sea surface temperatures have been higher during the past three decades than at any other time since large-scale measurement began in the late 1800s. From 1901 through 2009, sea surface temperatures rose at an average rate of 0.12 degrees Fahrenheit per decade. Over the last 30 years, temperatures have risen more quickly at a rate of 0.21 Fahrenheit degrees per decade.

Example of a sea surface temperature map based on satellite measurements and computer models. “Warm” colors such as red and orange indicate warmer water temperatures.
NASA, 2008

Three different studies show that ocean heat content (the amount of energy the ocean absorbs from the sun) has increased substantially since 1955. Ocean heat content not only determines sea surface temperature, but also affects sea level, currents, and the temperature on land.

Between 1993 and 2005 sea level rose, on average, 3 millimeters (0.1 inches) per year, attributed to an increase in melting ice and thermal expansion as the ocean absorbs more and more energy from the sun.

During the 20th century, sea level rose an average of 7 inches after 2,000 years of relatively little change. The 2007 IPCC report conservatively predicted that sea levels could rise 10 to 23 inches by 2100 if current warming patterns continue.

Past and projected global average sea level. The gray shaded area shows the estimates of sea level change from 1800 to 1870 when measurements are not available. The red line is a reconstruction of sea level change measured by tide gauges with the surrounding shaded area depicting the uncertainty. The green line shows sea level change as measured by satellite. The blue shaded area represents future global sea level projections. IPCC 2007

By looking at hurricane records, researchers saw that were almost twice as many Katrina magnitude events in warmer years than there were in colder years. Another study found that hurricanes, typhoons and tropical storms were intensifying more rapidly than 25 years ago.

The study is based on data from monitoring stations along the Eastern Seaboard of the United States, where the daily tide levels have been recorded all the way back to 1923. Rapid changes in sea level show that there has been a tropical storm. The map shows cloud cover and ocean temperatures when Hurricane Katrina hit New Orleans in 2005. Warm colors show ocean temperatures exceeding 28° C which can strengthen hurricanes.NASA/GSFC

A new study in the journal Science found that melting polar ice caps in Greenland and Antarctica raised sea levels by nearly half an inch over the last two decades. Combining satellite data from dozens of earlier studies, scientists not only have definitive proof that ice is melting, but that its happening much faster than expected.